The invention relates to a transmitted light refractometer that works according to the principle of a transmitted light refractometer. A light source illuminates a slit. Using optical components, the emitted light is directed as a beam and enters the process medium through a window. The light passes through the process medium as a parallel beam. The light refraction that is relevant for the measurement occurs when the light re-enters the refractometer through the measurement prism. Optical components produce an image of the slit on the light-sensitive position detector. The light refraction of the process medium is determined from the position determined for the slit image.
The working principle for refractometers has been known for more than one hundred years. In addition to various laboratory tasks, refractometers are increasingly employed in industry. Refractometers are used in the chemical, pharmaceutical, and foods industries for process control and/or online analysis.
In the past, critical angle refractometers have been used in process instrumentation (U.S. Pat. No. 3,628,867 A). These instruments use the principle of total reflection. An optical window is used to tie it to the process, e.g. U.S. Pat. No. 4,571,075 A. The light refraction relevant for the measurement occurs at the interface between the window (greater refractive index) and the process medium (lower refractive index). According to the critical angle principle, a beam strikes the interface with the greatest possible angular distribution. The part of the light having smaller angles of incidence is refracted in the process medium, while light having larger angles of incidence is increasingly totally reflected. A progression from light to dark is measured in the image plane. The refractive index of the process medium is determined using comparatively complex image analysis. In these refractometers, the practical access to the process on one side is advantageous and makes it possible to use them at various measuring points in pipes and tanks.
A broad and uniformly distributed angular spectrum for the light incident on the prism is required for covering the largest possible measuring area with high accuracy. Producing and imaging light with a broad and uniformly distributed angular spectrum is relatively difficult. In order to attain appropriate accuracy for the process, the possible measuring area is therefore generally limited. Therefore it may be necessary to combine a plurality of devices in order to cover the complete measuring area that is required from the process aspect.
The critical angle principle is susceptible to fouling. The interface between process window and process medium that is critical for the measuring effect can become fouled during the process and this fouling can stay undetected because the light is totally reflected at the interface. Regular cleaning and maintenance is necessary to prevent this.
Since critical angle refractometers usually evaluate the light/dark limit of the transition to total reflection, even minor shifts in the overall image on the detector lead to inaccuracy in determining the refractive index. Such shifts are caused for instance by changes in pressure and temperature on the sensor head. Special solutions attempt to render the optical arrangement more robust with respect to external influences (U.S. Pat. No. 6,067,151 A).
Overall, the aforesaid demanding process conditions for critical angle refractometers frequently lead to unintended drift effects in the measured value so that it is not possible to reliably accomplish the process objectives.
The principle of transmitted light refractometers is also known (see FIG. 1). In a transmitted light refractometer, the deflection of a parallel light beam that strikes the interface between the liquid 4 to be measured and the measurement prism 6 is measured, the light passing completely through the liquid in the measuring chamber 5. A slit 1 is illuminated by a light source. A beam is guided using optical components 2 and as a parallel beam 8 enters the process liquid from the interior of the refractometer via an optical window 3. The light passes through the process liquid and again strikes an optical component that forms the transition to the interior of the refractometer. This optical component is called the measurement prism 6. The parallel beam strikes the inclined interface and is refracted there. The deflection of the light beam is a function of the refractive index of the process liquid. Then the parallel beam is focussed onto the detector 9 using an optical component 7.
In a transmitted light refractometer, a sharp image of the slit occurs on the image plane of the detector. The detector is a light-sensitive position detector. The position of the slit image is determined by signal analysis. It is relatively simple to determine the angle of refraction and thus the refractive index of the process liquid from the position.gamma=ArcSin((n0/nRef)Sin(ArcTan(x/f)))  (I).Gamma is the angle of deflection after the light exits from the measurement prism.nRef is the optical refractive index of the measurement prism.x is the position of the slit image on the detector.f is the focal length from the imaging optics unit to the detector.n0 is the optical refractive index of air.beta=alpha gamma  (II).Alpha is the angle of incidence of the measurement prism.Beta is the angle of the refracted light beam in the measurement prism.nD=nRef*Sin(beta)/Sin(alpha)  (III)nD is the optical refractive index of the process medium.
The parameter x is measured, and the sought variable, nD, can be calculated from it when the device-related variables nRef, alpha, and f are known and when n0 is known.
Transmitted light refractometers are characterized by high measurement accuracy. Determining the position of a sharp image, e.g., of a thin slit, is relatively simple and is even possible without modern electronics. The precision and reliability of the position determined for the slit image can be further enhanced using modern signal processing methods. The angle of deflection, which is a function of the refractive index, can be determined very precisely, and thus the refractive index of the process medium can be measured with great accuracy.
Transmitted light refractometers require that light passes through the entire measurement chamber. This necessitates process access on two-sides if, as is usual, the light source and detector are arranged along one optical axis. In the process, a complex solution that has been structurally specially adapted to the process conditions must be found for each measuring location, so that installation is relatively complex. The aforesaid solution is only practical for smaller process tubes or laboratory applications; measurements in large vessels are not possible.
There are refractometers that evaluate images from two or more beams. These refractometers are called differential refractometers and can be created both as transmitted light refractometers (DE 4038123 A1) and in the form of critical angle refractometers (U.S. Pat. No. 3,449,051 A). The light emitted is divided into two beams that pass through the refractometer on different light paths. One beam travels a light path in which the light is diffracted at a reference liquid. This beam is called the reference beam. The other beam is refracted at the sample and is called the probe beam. The difference between the two images is evaluated in the detector.
Like mass density, optical refraction is a typical material parameter like, for example, density. Other material characteristics such as, for example, the mass concentration of the components in substance mixtures, can be derived from the measured refractive index. These derived variables are frequently the variables that are actually of interest in process instrumentation. Since the refractive index is also a function of media temperature, in general it is not possible to calculate the derived material variables unless the temperature is known (temperature compensation). There are different solutions for compensating the temperature correlation. In the case of critical angle refractometers, during the process temperature detectors are frequently built into the interior of the probe so that the temperature correlation can be corrected afterwards. These internal temperature detectors often do not satisfy the desired demands in terms of reaction time.
Without this additional temperature detector, the differential measuring principle can be used in a special embodiment for temperature compensation such as e.g. DE 3028564 A1. The reference liquid is selected such that its temperature behavior in terms of the refractive index is consistent with the behavior of the sample.
Thus, the measurement is temperature compensated. In this method the lack of flexibility and the structure, which is not appropriate for the process, are disadvantages. In addition, reference liquid and sample liquid frequently have only similar temperature behaviors, but not sufficiently identical temperature behaviors.
The underlying object of the invention is therefore to create a transmitted light refractometer that:
has high measurement accuracy across a broad measurement range, even under difficult measuring conditions;
can be connected to the process simply via a single access;
with a one device embodiment covers a measurement range for all practically relevant media; and,
has integrated temperature compensation.